1. Field of the Invention
This invention relates to a mechanism which can be incorporated in a drill string for effecting a change in direction of a rotary drill bit relative to the existing bore of a subterranean well.
2. Description of the Prior Art
The changing of the direction of drilling of the bore of a subterranean well is an expedient long practiced by well drillers. In many instances the change in direction is to produce a straightening of the well due to the deflection of the rotary drill from the desired direction by a particular rock strata. In other instances, the change in direction is intentional in order to reach a formation that is laterally displaced from the existing location of the bore hole.
One of the most common expedients for changing the direction of drilling has been the insertion in the drilling or work string, at a point above a downhole motor which drives the rotary drill bit, an apparatus which is called a bent sub. Such bent subs are rigidly connected at one end to the work string and have their other connecting end angularly disposed relative to the axis of the work string to which they are connected, so that when the motor and supported drill bit are rigidly supported thereby, the rotational axis of the drill bit will be angularly inclined relative to the axis of the well bore existing prior to insertion of the bent sub.
Because the change in angle has heretofore been accomplished at a substantial distance above the rotary drill bit, particularly with the class of fluid motors known as turbines, which by design are relatively long, a large degree of interference is created, between the bent sub and the rotary drill bit and the well bore wall as the work string is lowered in the hole to where the drilling would again begin. Additionally, to effect a desired change in drilling angle, which generally is on the order of a fraction of a degree, it was necessary to remove the motor and drill bit from the end of the work string and insert a particular bent sub which had the desired angular deviation incorporated therein. This required the maintenance at the drilling site of an inventory of bent subs having different deviation angles.
U.S. Pat. No. 4,522,272 discloses a bent housing for incorporation in a drilling string which is adjustable to provide a range of angular positions of the rotary drill relative to the axis of the drilling string.
A lesser known method for changing the direction of drilling, but an effective one, particularly with the aforementioned long turbine motors has been the incorporation of an eccentric stabilizer on the lower end of said turbines. This method is well documented by a presentation at the 1979 Drilling Technology Conference, Denver, Colo. in a paper titled "Turbo-Drilling Deviated Holes in Abu Dhabi" and a more recent article published in the October, 1982 issue of Journal of Petroleum Technology entitled "Kicking Off In Large Diameter Holes". This method has removed some of the interference problems of the bent sub but has required a large inventory of fixed, offset axis stabilizers, and has not addressed the problem of easily changing the amount of offset of the stabilizer depending on the current drilling and rig conditions.
There was therefore a recognized need in the well drilling industry for an apparatus which will permit a selected change in the drilling direction to be effected without the large degree of interference mentioned above, and secondarily, without having to maintain a large inventory of fixed offset stabilizers.
U.S. Pat. No. 4,560,013 provides an adjustable eccentric stabilizer mounted intermediate the motor housing and the rotating drilling bit for effecting a change in the direction of the drilling bit primarily by transversely shifting the rotational axis of the drilling bit. Such apparatus has been highly successful in drilling wells through a large variety of formations, but, as is well known in the art, directional drilling depends upon two primary factors: (1) the amount of interference with the side wall of the previously drilled bore developed by transversely shifting the axis of rotation of the rotary drilling tool or (2) a change in the angular direction of the axis of rotation of the drilling bit so that the face of the drilling bit proceeds in a different direction than the axis of the previously drilled portion of the well bore. A number of papers have been written on the subject and the general consensus is that for certain types of formations and hole sizes, directional change produced by side wall interference of the drilling bit with the bore wall is quite effective, while for other types of formations and other sizes of bores, better results are achieved by angularly changing the axis of rotation of the drilling bit.
The prior art has not heretofore provided a rotary drilling tool which can be adjusted to effect directional changes either by side wall interference or by changing the angular position of the axis of rotation of the rotary drilling tool, or through a combination of such adjustments.